Rena is the woman at the mill who points Coen toward Stoneward Post in The Firebrand. Play along and she quietly tells you Mert is held there for working with the Manumits.
Rena is a contact at the mill who knows what happened to Mert. She can't speak openly with soldiers around, so she needs you to go along with her act.
You reach her during The Firebrand, after searching Vladimir's hut. Clearing the soldiers at the mill and talking to her each give a little XP before you head to Stoneward Post.
Mert
Mert is a man held prisoner at Stoneward Post for working with the Manumits. Reach him in time during The Firebrand or he is executed, and freeing him opens the rebel storyline.
Vladimir
Vladimir is a friend of Coen and Pieter from Laslea and a member of the Manumits. In the prologue he plans to strike back at Brencis, and the rebel storyline picks up from his hut.